Document WAL rules related to PD_ALL_VISIBLE in README - Mailing list pgsql-hackers

Proposed wording attached.

The typical WAL rules are broken for setting PD_ALL_VISIBLE. I'm OK
with that -- rules are meant to be broken -- but it's confusing enough
that I think we should (internally) document it better. This doesn't
guarantee things won't change again in the future, but this behavior
has been stable for a while.

The thread here:

https://postgr.es/m/ee47ee24-2928-96e3-a2b1-97cbe07b2c7b%40garret.ru

also indicates that external projects and tools are relying on our
rules for the page LSNs.

Regards,
    Jeff Davis